Pulse modulation signalling system



April 1, 1947. D, l, 'LAWSON 2,418,268

PULSE AMODULATION VS IGNALL ING SYSTEM Filed Dec. 5l, 1945 2 Sheets-Sheet 1 F G, y ,l /a '/b l/nmodu/aed Wae/m l l' A l l, l/ l/ l,

F/ l ,l yl/ l/ i Waverm Mada/abbr? l/nmoou/Zated MMM/.1v www@ i sv@ I unuunnnnnu Modubted Pff/ses Inventor DL/IWSOM.

PHI 1, 1947. D. l. LAWSON PULSE vIVODUlLfA'I'IQN SGNLLING SYSTEM Filed Dec. 3l, 1943 2 Sheets-Sheet 2 Patented Apr. 1, 1947 rmtsEMoDULArIo sicNALLING SYSTEM Dennis Illingworth Lawson, Cambridge, England, assigner to Pye Limited, CambridgaEngland, a

British company s Application December 31, 1943, Serial No. 516,491 in Great Britain December 18, 1942 f 9 claims. I (C1. 417a-17H15) The present invention relates to a system of modulation fora signal-transmission system in which the transmission takes place in a series of pulses, particularly for ultra-high frequency radio transmission employing, for example, centimetre waves. Hitherto the modulation of such signals has been effected by varying the pulse Width.-

It has also been proposed to modify the usual form of pulse width modulation by transmitting extremely short pulses to mark the beginning Vand end of each of the pulses of variable duration, which systems are known as double pulse systems, Whenthe beginning of the variable Width pulses takes place at equal intervals of time or when the end A of the variable width pulses takes place at equal intervals of time, the short duration pulses corresponding to the beginning or end of the variable width pulses, whichever is equally spaced in time, (hereinafter called marker pulses) may be eliminated at the transmitter and re-inserted again at the receiver. United States Patent No. 2,256,336 describes such systems and a method of generating the pulses of a double pulse system by means of a cathode ray tube, some of the embodiments generating only the short duration pulses corresponding to the end of the variable width pulse which varies in time. In all double pulse systems previously proposed, however, where the short duration fixed interval marker. pulses are suppressed, it has been lnecessary to provide at the receiver for re-inserting the xed interval marker pulses so that Athe waveform which is de-modulated corresponds mitted. The amplitude of the modulation determines the amplitude of the phase displacement -of the pulses. One laulvantage of this type of transmission is that the signals may be received and demodulated without having to insert any xed interval marker pulsesat the receiver, as

was necessary in the prior'systems above referred to. i

A feature of the invention consists in fan arrangement for generating frequency modulated .pulses according to this invention which consists in triggering` the pulse generators byf ,means of n 2 n a synchronising voltage upon which is super- -imposed the modulations. `The synchronising wave may be amplitudemodulated by the modulation, or, alternatively, the synchronising` voltage and the modulations canbe fed to the same or separate control grids ofthe triggering .valve of the pulse generator. In either case them-.- stant' of triggering depends upon the instantat which the summation of the synchronising and modulation voltages reaches a suiiicientvoltage to trigger the pulse generator. i i

The invention will be more clearly understood by'making reference to the accompanying drawings, in which i Fig. l 4shows the wave form series of pulses.

Fig. 2 shows the wave `form of 'the same series of pulses modulated in accordance with. this invention. Fig.` 3 shows the modulation corresponding to the wave form of Fig. 2.

Fig. 4 shows one method of generating the series of pulses. y

Fig. 5 shows the method of modulating t he pulses. V Fig. 6 shows the circuit of a transition oscilla# tor modied for generating frequency modulated pulses in accordance with this invention. f

Fig. 7 shows a circuit arrangement for carrying out the invention using a multi-vibrator.

Fig. 7A shows curves of the voltagesgappearfing atfthe anodes and grids of the `valves in'the circuit shown in Fig. '7. y

Fig. 8 shows a further circuit arrangement for carrying out the invention using a" Kipp relay. Fig. 1` shows a series of pulses l, la, Ib etc., of constant width and constant spacing.The system of modulation according tothis invention consists in shifting the phase of the pulses in accordance with the modulation.

Fig. 2 shows by way of `example the pulses Adisplaced in phase corresponding to the sinusoidal modulation voltage showin in Fig. 3. Itwill be seen that `the time interval between successive condensations and'rarefactions of the pulses is varied sinusoidally at the frequency of the modu# lation to .be transmitted, whilst the amplitude of the. modulation determines the amplitude of the phase displacement of the individual pulses.

of anunmodulated TheA system of modulation according to this `invention gives an improved signal to noise ratio comparedwith present amplitude modulated systems,Y Since, noise voltages along g the horizontal portions of the waveform may be limited, for example, by a'diode, leaving noise only over the vertical regions of the waveform.

The method of producing the phase or frequency modulated pulses as above described will be explained with reference to Figs. 4 and 5,. It is well-known that various types of .relaxation oscillators, multi-vibrators, trans'itron oscillators and back coupled two stage ampliers in which the valves alternately conduct, commonly designated as flip-flop circuits, may be vmade to generate pulses of short duration and that these pulses may be further sharpened'by connecting a damped tuned circuit in the anode of the pulse generator. All these generators may readily be synchronised to any periodically Vvarying voltage connected to a control grid. Thus,.:f.or.examp1e, if a sinusoidal Voltage having a frequency of say 6 kc. is fed to a control-grid of the oscillator, the

modulated, then `the vrepetition 'frequency of the pulses will vary in a periodic manner asshown in Fig. in accordance-with the modulation frequency, the'rcpetition rate of the'pulses varyingwith the depth of-theamplitude modulation and the synchronising level.

It is not vnecessary that 'the synchronisingwave should be amplitude modulated; the synchronisin'gvvolta'ge :andthe modulation could be fed to twoseparate contrnl'grids 'ofa Valve of the oscillation generator or even to the same control grid- The `signals transmitted will comprise a tone due to the synchronising;frequency and the receiver should therefore beprovided with means for eliminating this tone.

Figs. 6, 7 and 8'show circuit arrangementsfor producing the phase modulated pulses according .to this invention. In Fig. 6 a circuit is shown which is generally kno-wn asa transitron .oscillator andcomprises the pentode valve V having aA-cathode-I, `an anode 2 .and three grids, namely, the control vgrid-3, .the screening grid 4 and suppressor grid .5 .arranged therebetween. Thezpositive high tension is fed to the anode through the resistance R3 connected at the point ofjunction cftheresistance RI and R2 shunted across .the high tension supply, the resistance R2 being shunted by a condenser CI. The screening `grid potentialis fedvia the resistance R8, the'screening grid .also being connected through the condenser C2'to`the`junction point of the resistances R5, R6 connecting the suppressor grid5 to the negative high tension lead. The control grid is connected to the cathode through the'resistance lThe oscillations inthe valveare maintained by virtue of the fact that the suppressor *voltage- Ascreen current characteristic curves of pentode valves exhibit negative-resistance properties. The oscillator-'may be, synchronised to Aany external voltage by applying thisto either the suppressor orcontrol gridel'ectrodes In Fig. 6 the sinewave I Asynchronising"voltage is applied to the suppressor grid 5 through the resistance R1 and the instant at which the valve is triggered is varied'by apply ing the modulation to the controlfgrid 43- ofthe valve through the condenser C3 and a resistance RIU. The current in the anode circuit of the valve consists of a series of sharp pulses which are phase modulated. In the circuit shown these pulses are further sharpened by passing them through the heavily damped tuned circuit LRS located in the cathodecircuit of the valve.

Fig. .7 shows an alternative circuit arrangement for producing phase modulated pulses by means of a multi-vibrator. The multi-vibrator consists of va two stage resistance coupled amplifier, with feedback'from the anode of Valve V2 applied to the grid of valve Vl. As the phase shift in each valveis the total phase shift for the two va1vesis.360,i. e. the feedback is in the correct phase for `the oscillation to be maintained. The operation of the circuit may be described as follows. Consider VI initially in a, non-conducting condition, then as soon as VI conducts, the anode of Vl and hence the grid of V2 will fall in potential, and the vvalve V2 Will become non-.conducting, and will remain non-conducting unti1 the .chargehas leaked away from the condenser C2 via the resistance R2 so that the grid potential of V2 can rise to a point at which the valve conducts. As soon as V2.conducts, the anode potential of V2 Will fall sending thegrid of VI negative. The potential of the grid of VI will rise as the charge leaks away from vthe condenser Cl via ythe resistance Rl, and eventually VI will Yagain conduct. The waveforms at the various electrodes are shown in Fig. 7A. It will be seen that if 'a periodic voltage o'f a slightly higher frequency than the free running frequency .of the multi-'vibrator is applied to the control grid, the frequencyof the .multi-vibrator will be locked to `the incoming frequency, efor one cycle of the multi-vibrator will commence always when the voltage of the locking oscillation has risen to a given value. The locking oscillation may be applied in the co-ntrol grid or the cathode circuit.

Inthe embodiment shown the synchronising -voltage is applied to the grid and the modulation in'thefcathode circuit of the Valve VI. Thus the instant at which VI becomes conducting will depend on the modulation voltage applied i. e. the output pulse will be phase modulated. It willbe seen that the modulation and synchronising voltages may be interchanged. Also other electrodes rofLrnulti-electrode valves could be used for applying .the locking voltages.

Fig.l 8 shows a further .circuit arrangement comprising a Kipp relay circuit for generating thegphase modulate-d pulses. The Kipprelay will notoperate continuously without a locking Pulse, as was the case 4with the multi-vibrator. Considering the cycle of events from the conditions Where valve VI is non-conducting and valve V2 is conducting. As soon as a positive input voltage is applied to the grid of Vl, the valve Vl will conduct and the anode voltage cf Vl will consequently fall. This will send the grid of valve V2 negative and V2 will cease to conduct. V2 will commence to conduct when its grid voltage which is rising at a rate determined by the .Values .of the condensers C and resistance R is only slightly negative. voltage of VI and V2 is increased, this decreases the anode current of VI .and the anode voltage of VI rises. This is transferred to the grid of V2 via the condenser C and the current taken by V2 is'further increased, the current in Vl falling by v y -virtue of the increasing cathode voltage. In'the fendfwe areleftwith V2 conducting andVl nonlconductingfas inthe initial condition.

As soon as V2 conducts the cathode- The instant at which the Valve Vl commences to conduct will depend on the voltage between its grid and cathode. The oscillation may therefore be synchronised with any input applied to the grid of Vl. The point at which synchronisation take place may be varied by applying the modulating voltage to the grid leak as shown. Obviously the modulation and synchronising voltages may be interchanged, or the voltages may be applied to other electrodes of multi-electrode Valves.

I claim:

1. A transmitter for a communication system comprising a valve oscillator for generating a series of constant width pulses and means for controlling the oscillator by the modulation to be transmitted so as to Vary the time interval between the pulses periodically in accordance with the frequency of the modulation.

2. Apparatus comprising a Valve oscillator which generates a series. of constant width pulses and means for triggering the oscillator and controlling the instants at which the pulses are produced by means of a periodic synchronising voltage upon which the modulation is superimposed.

3. Apparatus as claimed in claim 2, wherein the synchronising voltage is amplitude modulated by the modulation.

4. Apparatus as claimed in claim 2, wherein the pulse generator is triggered by the synchronising voltage and the modulation which are fed to the same means for triggering the pulse generator.

5. Apparatus as claimed in claim2, wherein the pulse generator is triggered by the synchronising voltage and the modulation which are fed to separate means for triggering the pulse generator.

6. Apparatus comprising means for generating constant width current pulses and means for applying a synchronising voltage and a modulation voltage to the pulse generator so as to trigger the pulse generator at the instant at which the summation of the synchronising and modulation Voltages reaches a predetermined Value.

7. Apparatus comprising a valve circuit for generating pulses of short duration and constant width, the repetition frequency of which may be synchronised to a periodically varying Voltage fed to one of the control electrodes of the Valve, and means for feeding a synchronising Voltage and a modulation voltage to at least one of the control electrodes of the valve to vary the frequency of the pulses in accordance with the modulation.

8. Apparatus comprising a transitron oscillator comprising a generator valve having an anode, a cathode and three control electrodes arranged therebetween, means for feeding a, synchronising voltage to one of said control electrodes and a modulation Voltage to another of said control electrodes to produce in the anode circuit of the valve a series of sharp constant width pulses which are modulated in frequency in accordance with the modulation.

9. A pulse modulation signalling system comprising a valve oscillator for generating a series of constant width pulses, means for applying modulation to the generator to generate the pulses at time intervals which Vary periodically in accordance with the frequency of the modulation to be transmitted, means for transmitting the modulated series of pulses.

DENNIS ILLINGWORTH LAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,171,150 Shelby ..v Aug. 29, 1939 2,256,336 Beatty Sept. 16, 1941 2,262,406 Rath Nov.. 11, 1941 2,272,070 Reeves Feb. 3, 1942 2,266,401 Reeves i Dec, 16, 1941 Disclaimer 2,418,268.-Denm`s I. Lawson, Cambridge, England. lULsE MODULATION SIGNAL- 947. Disclaimer led May 16, 1949,

LING SYSTEM. Patent dafued Apr. 1, 1

by the assignee, Pye Limited. Hcreby enters this disclaimer to claims 1 and 9 in said patent.

[Ojcal Gazette June 21, 1949.] 

